1. Field of the Invention
This invention is related to a process for preparing metallic sulphates. More specifically, this invention provides an improved process for preparing metallic sulphates from mixing of metallic sulphide ores, such as sphalerite, galena, chalcocite, pyrite, etc. with ammonium sulphate.
2. Description of the Prior Art
Production of metallic sulphates, such as zinc sulphate in calcining furnaces, is generally old in the art and has been disclosed in my U.S. Pat. No. 2,006,259 patented June 25, 1935, No. 2,074,210 patented Mar. 16, 1937, No. 3,443,888 patented May 13, 1969, and No. 3,672,830 patented June 27, 1972, all of which will be incorporated into this application. These patents disclose a cyclic process wherein when crystals of ammonium sulphate are calcined with zinc sulphide ores, the reaction commences at a temperature between about 150.degree. C. and 200.degree. C. with a slow fusion and decomposition of ammonium sulphate to ammonium bisulphate and ammonia as represented by Reaction (1). ##STR1##
As the temperature approaches 200.degree. C. the ammonium bisulphate begins to react with zinc sulphide to produce an anhydrous zinc sulphate sinter residue and vaporous ammonium products, such as S, SO.sub.2 and NH.sub.3, as illustrated by the following reaction: ##STR2##
The volatile S, SO.sub.2 and NH.sub.3 are dissolved in water and purified to produce solutions of ammonium thiosulphate and ammonium hydroxide.
When the temperature is maintained at approximately 440.degree. C. for about 10 minutes all of the ammonium bisulphate has reacted with zinc sulphide and only anhydrous zinc sulphate and unreacted zinc sulphide remain. The anhydrous zinc sulphate residue, or sinter, is dissolved in water, purified and produces a concentrated solution of zinc sulphate which when combined with the ammonium thiosulphate and ammonium hydroxide produces zinc sulphide pigments and ammonium sulphates in accordance with the equation:
ZnSO.sub.4 + (NH.sub.4).sub.2 S.sub.2 O.sub.3 + 2NH.sub.4 (OH).fwdarw.ZnS + 2(NH.sub.4).sub.2 SO.sub.4 + H.sub.2 O (3)
thus, a cyclic process is provided when ammonium sulphate of Equation (3) is recovered and returned to react with the zinc sulphide of Reaction (2). The preferred specific embodiment of this cyclic process was fully disclosed and patented on May 13, 1969, in my U.S. Pat. No. 3,443,888. The sinc sulphide pigment of Equation (3) may subsequently be washed, dried, and dehydrated by calcining at a low temperature to produce a superior zinc sulphide pigment as disclosed and patented on June 27, 1972, in my U.S. Pat. No. 3,672,830.
It has been discovered that the reaction of Equation (3) can be controlled precisely and efficiencies of 95% or higher can be obtained. However, Reactions (1) and (2) are beset with many interfering side reactions that result in low efficiency in the production of zinc sulphate and objectionable by-products in the gaseous products which tend to throw the cyclic process out of balance and greatly increase the cost of operation.
If air infiltrates into a calcining furnace while a mixture of a metallic sulphide ore, such as zinc sulphide, and ammonium sulphate are being treated, a portion of evolved elemental sulphur is oxidized as shown in Equation (4) below resulting in ammonium thiosulphate and ammonium hydroxide recoveries being reduced and a build up in the system of ammonium sulphite. EQU 2(NH.sub.4).sub.2 SO.sub.4 + ZnS + 1.50.sub.2 .fwdarw.ZnSO.sub.4 + 2(NH.sub.4).sub.2 SO.sub.3 ( 4)
the ammonium sulphite has to subsequently be removed and additionally treated with elemental sulfur for conversion into ammonium thiosulphate before it can be used for production of zinc sulphide pigments as aforementioned. Infiltration of air can be corrected by calcining in a tight furnace or a retort externally fired which has been provided with seals to prevent ingress of air, but these furnaces must be manufactured with expensive alloys and are susceptible to corrosion by the ammonium acid sulphate produced from the fusion and decomposition of ammonium sulphate of Reaction (1). The fused mixture forms a viscous pasty mass which remains semi-fluid and sticky until Reaction (1) is more than half complete. This necessitates the use of tumbling rails within the calcining furnace in order to break up the sticky, pasty lumps and keep the charged mixture moving through the furnace.
Another objectionable side reaction is the sulphiding of the newly formed zinc sulphate with the elemental sulfur of Reaction (2) in accordance with the following equation: EQU ZnSO.sub.4 + 2S.fwdarw.ZnS + 2SO.sub.2 ( 5)
this side reaction develops quickly as the temperature approaches 300.degree. C. because sulphur is a liquid at this temperature. Liquid sulphur prolongs the semi-fluid condition of the pasty charge to additionally hinder the progression of the charge through a calcining furnace. This side reaction also reduces the production of zinc sulphate, or any metallic sulphate of the corresponding metallic sulphide ore, and produces an excess of ammonium sulphite in the volatile products.
The extent of which all of these hereinbefore side reactions throw the cyclic operation out of balance may be indicated by comparing the equivalents of free ammonia and the elemental sulfur collected with the water soluble zinc sulphate produced. If the molar ratios of ammonia to zinc sulphate, or sulphur to zinc sulphate, equal unity then the cyclic process is 100% efficient. In practice wich conventional processes, the efficiency for producing zinc sulphate varies from between 0% to 80%.
Therefore, what is needed is an improved efficient process for preparing metallic sulphates from the mixing of metallic sulphide ores with ammonium sulphate which overcomes all of the foregoing deficiencies and side reactions associated with prior art processes. More specifically, what is needed is an improved process for preparing zinc sulphate which is particularly adapted for combination with the known apparatuses and processes of refining zinc sulphate in accordance with Reactions (1) and (2).